(a) Field of the Invention
The present invention relates to a membrane electrolysis stack, an electrodialysis device including at least one such membrane electrolysis stack, and a method for the regeneration of an electroless plating bath in which such an electrodialysis device is employed for regeneration.
(b) Brief Description of the Prior Art
Electroless metal plating baths, for example electroless nickel plating baths, contain reducing agents, for example hypophosphite, to deposit the metal on the surfaces to be metallized. The reducing agent is oxidized during the deposition and is no longer available for the rest of the metal plating process. In the case of electroless nickel plating baths operated by means of hypophosphite, orthophosphite is formed in this way from hypophosphite. Furthermore, the counter-ions of the metal salts used for this are additionally introduced into the bath during the replenishment of the metal consumed during the metal plating. In the case of a nickel plating bath, sulfate anions are supplied to the bath, for example. Like the oxidation products of the reducing agent, these ions accumulate in the bath and interfere with the rest of the metal plating process. The service life of an electroless plating bath is due to the accumulation of these reaction products thus normally limited. To extend the service life of such baths, many suggestions have been made.
To this end, for example, electrodialysis devices are known which have at least one anode and at least one cathode as well as a respective membrane electrolysis stack arranged between an anode and a cathode. Each membrane electrolysis stack comprises membranes stacked on top of each other and respective electrolyte compartments arranged therebetween. The membranes comprise first and second membranes in alternate succession in the membrane electrolysis stack. Additionally, methods for the regeneration of an electroless metal plating bath are known in which the bath is transported into diluate compartments of an electrolysis device which has at least one anode, at least one cathode and at least one membrane electrolysis stack, and is freed therein from interfering ions present in the bath by transferring the interfering ions by means of electrodialysis from the diluate compartments to concentrate compartments likewise present in the electrodialysis device.
For example, a method for the regeneration of aqueous, electroless plating baths for the deposition of metal by means of metal ions and by means of a reducing agent is known from DE 43 10 366 C1, in which the plating bath to be regenerated contains Cu, Ni, Ag or Au metal ions, consumed and optionally unconsumed hypophosphite as the reducing agent, the counter-ions respectively required for this and optionally further components, the plating bath is directed into one or more electrodialysis cells equipped with cation and anion exchange membranes and its anode compartment is limited by a cation exchange membrane, consumed and unconsumed hypophosphite is further removed from the plating bath via an anion exchange membrane by electrodialysis and directed into the cathode compartment, consumed hypophosphite is further regenerated in the cathode compartment by reduction and regenerated and unconsumed hypophosphite is finally removed from the cathode compartment and supplied to the plating bath via an anion exchange membrane by electrodialysis.
Furthermore, a method for the regeneration of a process solution is known from EP 1 006 213 A2, the process solution being used in the chemical reductive deposition of metal layers and containing hypophosphite and orthophosphite, in which method the process solution is filled into an electrodialysis cell with at least four compartments, the electrodialysis cell having an anode with an anode compartment, a cathode with a cathode compartment and two more compartments separated by an anion exchange membrane and arranged between the anode compartment and the cathode compartment, a first compartment of these being separated from the cathode compartment by an anion exchange membrane while a second compartment is separated from the anode compartment by a cation exchange membrane, the process solution is filled into the first compartment when performing the method whereby the hypophosphite ions and orthophosphite ions contained therein are electrodialytically transported into the second compartment and at the same time hypophosphite ions are transported from the cathode compartment into the process solution, and regenerated process solution is extracted and supplied to another use, wherein the acid mixture formed in the second compartment by the influx of protons from the anode compartment is supplied to the weakly alkaline anion exchanger present in the hypophosphite load, the outlet of which being connected to the cathode compartment.
Furthermore, DE 10 2004 002 778 B4 discloses a method for the regeneration of electrolyte baths used for electroless metallization. According to this, at least a partial flow of the electrolyte is initially drained from the process container, the drained electrolyte flow is then regenerated and the regenerated electrolyte flow is finally recycled into the process container. The drained partial flow is supplied to a dialysis and/or electrodialysis unit for regeneration in which the anions released in the electroless metallization process are exchanged via an anion-selective membrane. Furthermore, a solution containing alkaline and/or alkaline earth hydroxide is employed as a counter solution for the dialysis and/or electrodialysis of the electrolyte. Components consumed in the metallization process are added to the electrolyte prior to the recycling into the process container. The alkaline and/or alkaline earth hydroxide solution employed in the dialysis and/or electrodialysis process is regenerated after the dialysis process.
A method for the regeneration of an electroless nickel plating bath is known from U.S. Pat. No. 5,419,821 A, in which anions of the bath within an electrodialysis device are transferred via an anion exchange membrane into a compartment within the device by electrodialysis into which protons are transferred from an anode compartment through a cation exchange membrane. The compartment including the bath is also separated from the cathode compartment by another anion exchange membrane.
A method for the electrodialytic regeneration of an electroless metal plating bath containing hypophosphite ions as the reducing agent is specified in EP 1 123 424 B1, in which the liquid of the bath is directed through diluate compartments in a first electrodialysis device having cathodes and anodes wherein these diluate compartments are separated from concentrate compartments in this electrodialysis device on the cathode side by monoselective cation exchange membranes and on the anode side by anion exchange membranes, wherein the diluate compartments and the concentrate compartments are arranged alternately with regard to each other and the bath liquid is simultaneously directed through diluate compartments in a second electodialysis device having cathodes and anodes, the diluate compartments being separated from concentrate compartments in the second electrodialysis device on the cathode side by monoselective anion exchange membranes and on the anode side by anion exchange membranes, the diluate compartments and the concentrate compartments in the second electrodialysis device being arranged alternately with regard to each other.
A method for the regeneration of an electroless metal plating bath is likewise specified in EP 1 532 295 B1 comprising the following method steps: a) directing the metal plating bath through respective diluate compartments of electrodialysis devices, b) directing a concentrate liquid serving for the removal of interfering substances to be removed from the metal plating bath through respective concentrate compartments of the electrodialysis devices, these concentrate compartments being separated from the diluate compartments by ion exchange membranes, and c) directing the concentrate liquid also through main cation exchangers and back into the concentrate compartments by enabling the concentrate liquid to circulate in a first circuit between the concentrate compartments and collection containers and in a second circuit between the collection containers and the main cation exchangers.
At least with the methods mentioned last, the orthophosphite ions and sulfate ions as well as further interfering cations, such as sodium ions, generated during the metal plating can be removed from the metal plating bath while the recyclable substances, for example the hypophosphite ions, carboxylic anions and nickel ions, remain in the bath.
EP 0 416 312 A2 teaches a method of continuously removing and obtaining ethylene diamine tetraacetic acid (EDTA) from the process water of electroless copper plating. For that purpose the process water containing the EDTA is directed into an electrodialysis cell alternatingly equipped with bipolar membranes and anion exchange membranes and/or cation exchange membranes, and to which a potential difference is applied. The EDTA is converted into its charge-neutral form by means of protonation, and the protons required for that purpose are generated via bipolar membranes through electrodialysis. The required pH-value is adjusted by a pH-controlled electric field regulation via the bipolar membranes.
Further, U.S. Pat. No. 6,294,066 B1 teaches an apparatus and a process for the electrodialysis of salts. The basic electrodialysis apparatus is a cell having a number of compartments separated by membranes. A DC source is connected to drive a current through a feed stream passing through the cell which splits the salt stream into an acid and a base. The electrodialysis cell comprises in a first embodiment a stack of bipolar and anion membranes, in a second embodiment a stack of bipolar and cation membranes and in a third embodiment a stack of bipolar, anion and cation membranes.
However, it has been found that problems often occur in the electrodialysis devices due to biological fouling as the bath liquid contains components in substantial amounts allowing for the existence of microorganisms, for example organic carboxylic acids. The biofilms generated in the course of this consist of microorganisms, such as fungi and bacteria. These films cause hydraulic and electric resistances in the devices, i.e. they clog the electrolyte compartments and the inlets and outlets and lead to electrical problems by increasing the resistance throughout the individual cells. This can result in the membranes being burned and the entire system failing. Thus, the cells have to be cleaned frequently. However, even when the electrodialysis devices are cleaned regularly a relatively short service life of the membranes results.
Efforts to prevent the generation of the biofilms by using biocidal substances were indeed partially successful; however, they were associated with detrimental effects on the environment, health, properties of the deposition process or the deposited layer.